A wing’s primary function is the generation of aerodynamic lift. Hence it may be considered as the most important component by a design engineer if the aircraft is to perform its most fundamental function and achieve flight.
For such reason, a vast array of different wings have been developed , with each new model refined to suit the specific operational requirements of that aircraft. In the same way the delicate wings of a hummingbird are vastly different to the magnificent wingspans of an eagle, you would not expect particularly great performance to arise if the same wings of a civil aircraft carrier were applied to the design of a fighter jet. This article will outline the key design parameters to be considered and provide general guidelines as to the most appropriate choices with respect to different performance and mission requirements.
Conceptual Wing Design
The conceptual design phase in aviation aims to generate a high-level visualisation of the desired configuration, from which all proceeding decisions and calculations will ensue - in essence what will the wing actually look like. This includes the determination of the number of wings, vertical location, and basic geometry. The answer to these questions lies in the dictated mission requirements stemming from both the customer and the appropriate regulatory body. For example, a high wing is laterally more stable, whilst a low or mid wing have greater manoeuvrability at the expense of this stability. This is only one of the many aspects to consider when choosing the vertical placement, but in general cargo aircraft tend to opt for high wings, passenger aircraft are designed with low wings, and the mission requirements of fighter aeroplanes align most often with mid wing characteristics.
Figure 1: An agricultural sesquiplane in operation
The choice of the number of wings is perhaps the easiest decision for designers since, nowadays, a single wing is the most aerodynamically efficient and greatly reduces complexity. In the past, multiple wings were selected due to manufacturing limitations which could not structurally support large wing spans, and hence to overcome this, multiple wings were opted to produce sufficient lift. However, these restrictions have long since been conquered by advanced technologies and materials, and therefore the lower weight and greater pilot visibility of single wings have pushed monoplanes to dominate as the practical choice in the majority of cases. Having said this, there is one specific market for which two wings have demonstrated the upper edge. Sesquiplanes are characterised by the shorter span of their lower wing, a distinctive feature which results in four wing tip vortices, a phenomenon which has been utilised by the agricultural industry for the effective spreading of fertilizer and insecticide.
Wing geometry encompasses the taper ratio and sweep angle. Alterations in the taper ratio, defined as the ratio between the tip chord and root chord, can primarily result in three different planforms: rectangular, trapezoidal and delta shaped. Although a rectangular wing is cheaper to manufacture and is concurrent with a high stalling angle of attack due to a greater downwash angle at the tip in comparison to the chord, this planform is inefficient in terms of structural weight and induced drag. Varying the taper of the wing provides a means of achieving elliptical lift and load distributions which incur aerodynamic and structural benefits respectively. Alterations to the taper ration can also result in greater lateral control, the optimal configuration for which is a delta shaped profile. With respect to the sweep angle, it can generally be observed that low subsonic aircraft tend towards a straight wing, whilst most high subsonic and supersonic aircaft will opt towards a swept design. The reason being that one of the main benefits of a swept wing is to delay compressibility effects which occur above flight speeds of 0.3 Mach and hence, for low subsonic flight, the induced cost and complexity outweighs any minimal increase in performance.
As the name would suggest, the detailed design builds on the high level model generated by the conceptual phase, to quantify exact values for the taper ratio and sweep angle in addition to multiple other parameters such as the aspect ratio and aerofoil section.
The aspect ratio is deserving of greater discussion since it has one of the greatest impacts on the success the aircraft with respect to its intended use, and therefore great variance can be observed within aviation. In the one corner, a higher aspect ratio boasts a greater lift coefficient, lower induced drag and greater lateral control whilst in the opposing corner, a lower value results in lower weight and greater roll manoeuvrability. In general, higher aspect ratios are aimed for by general aviation and to an even greater extent by gliders (the Socata HALE reaching a value of 32.9), whilst fighter aircraft exploit the high manoeuvrability of aspect ratios around 2-4.
Finally, no guide on the various wings employed by aviation would be complete without reference to the aerofoil. The greater budgets of global aerospace companies may stretch to the creation of a new aerofoil for each new project, but in general the extensive databases of historical aerofoils, such as NACA, are drawn upon. Of these, the most widely applied are the 6-series NACA aerofoils. The five numbers which define each member of this family represent, in sequential order, the NACA series (6), chord wise position of the minimum pressure, ideal lift coefficient and the last two indicating the maximum thickness to chord ratio. For aircraft aiming to achieve high subsonic speeds, it is recommended to opt for a lower thickness to chord ratio since a thinner aerofoil will delay the onset of drag divergence, allowing the plane to fly closer to Mach one.
Recent Developments in Aviation Wings
A recurring theme in this article has been the constant compromise between the different performance characteristics offered by varying wing designs. However, a team of NASA and MIT researchers may have finally found a solution to this long-founded frustration with the development of their morphing “metamaterial” wing. The concept is to manufacture the wing from a lattice like structure of minute, identical polymer tiles featuring struts along each edge, and covering the ensuing framework in a similar polymer skin. By matching the flexibility and location of the struts to the expected loads, the material automatically reconfigures itself into the desired form this optimising to any encountered operation. Furthermore, the density of this intricate structure is approximately 0.3% that of conventional wing material, incurring drastic weight reduction.
The concept of flexible operation is also a hot topic for spaceflight company Virgin Galactic in their continuing development of the SpaceShipTwo, a commercial, passenger carrying spaceship. Achieving a single wing configuration for both space flight and conventional runway landing would to many appear an improbable feat. This may perhaps still stand true if the word single is adhered to strictly, since Virgin Galactic’s innovative solution is to create a rotating wing and rudder system allowing rapid conversion between two distinctive configurations. Whilst the spacecraft rockets out of Earth’s atmosphere, the more streamlined wing profile as can be seen in Figure 2 is employed; a design akin to that employed by most conventional space shuttles. However, upon re-entry the wing and tail boom revolve upwards into their “feathering” position. The phrase feathering is particularly appropriate since the effect achieved mimics the dynamics of a simple shuttlecock, harnessing aerodynamic forces to control the stability and rate of deceleration, in order to safely transport passengers back into the atmosphere. At 70000ft the structure defeathers into a gliding configuration, allowing smooth approach to conventional runway landing.